The invention relates to a method and system for measuring pressure and/or other physical data, such as temperature and deformation due to axial, torque and/or bending stresses by means of one or more optical fibers that are embedded in a deformable carrier tube.
Various fiber optical systems are known for measuring pressure and/or temperature over long distances.
Japanese patent publications JP6043056, JP9026370, JP6148017, JP60219503, JP2000-018981 disclose fiber optical pressure sensor systems, wherein a coiled optical fiber is embedded in a flexible tubular or cylindrical carrier body such that deformation of the carrier body will cause variation of the curvature and shape of the coiled optical fiber. This deformation initiates variation of transmission losses in optical signals transmitted from one end towards another end of the coiled fiber. These known systems are configured to detect an average pressure exerted on the carrier body and are not designed for use as Distributed Pressure Sensors (DPS) which detect the pressure exerted at different locations along the length of the carrier body and map pressure variations along the length of the carrier body.
U.S. Pat. Nos. 5,845,033 and 6,233,374 and International patent applications WO 00/00799 and WO 01/07935 disclose coiled optical fibers inscribed by Fiber Bragg gratings for detecting pressure exerted to a carrier tube in which the coiled optical fibers are embedded. A Fiber Bragg grating is formed by a pair of axially displaced disruptions in the fiber, which will reflect a narrow band of light at a specific wavelength. Variation of the spacing between a pair of such disruptions, for example due to strain induced by pressure and/or temperature, will cause a shift in the wavelength of the light it reflects. Thus a detected shift in wavelength reflected by a Fiber Bragg grating can be correlated to an elongation or shortening of the length of the optical fiber between a pair of the disruptions that form the grating. Various Fiber Bragg gratings may be distributed along the length of an optical fiber and the signals reflected by these grating may be multiplexed and distinguished from each other by known interferometric signal distinguishing techniques. Fiber Bragg grating optical sensor systems can be used for distributed pressure measurement along the length of an elongate carrier tube, but the currently available signal distinguishing techniques limit the amount of Fiber Bragg gratings that can be distributed along the length of an optical fiber. Furthermore Fiber Bragg gratings provide fixed sensor sections in an optical fiber, so that pressure and/or other measurements can only be performed in pre-selected section of the fiber, which are subsequently positioned at pre-selected locations where the measurements are to be made.
UK patent application GB2303445 discloses a hydrophone comprising a coiled optical fiber, which is embedded in a flexible conduit that is arranged between two rigid flanges. If an isotropic pressure is applied to the fiber, this causes an anisotropic stress vector in the optical fiber, which may be detected using birefringence in the optical fiber. Thus the known hydrophone is configured to detect variation of the isotropic pressure in the water surrounding the hydrophone and is not suitable for use as a DPS. U.S. Pat. Nos. 5,765,948 and 5,825,804 disclose fiber optical Distributed Temperature Sensor (DTS) systems wherein solely the temperature distribution along the length of an optical fiber is determined on the basis of optical time domain reflectometry, wherein use is made of the Raman spectrum of the backscattered light, which contains temperature information.
European patent application 0377549 discloses a combined DPS and DTS fiber optical system, wherein use is made of a fiber that has a central light guiding region into which interrogating light is launched and a peripheral light guiding region comprising fluorescent material into which a relatively large fraction of light from the central region is deflected under the action of force exerted to the fiber. The light transmitted by the fluorescent material has a unique wavelength, which is different from the wavelength of the interrogating light. By using a pulsed interrogating light source and by measuring the time of flight of the light transmitted by the fluorescent material the location(s) at which force is exerted to the fiber can be detected.
International patent application WO 98/27406 discloses a combined DPS and DTS fiber optical system in which the wavelength of backscattered light is used to measure both temperature and pressure at any point along the length of an optical fiber. The wavelength of the backscattered light changes depending on the strain and temperature at the point from which it is backscattered, which is known as the Brillouin shift. By accurately measuring both the amplitude and the frequency shift of the Brillouin scatter light and comparing it with a reference fiber, both temperature and strain can be measured simultaneously at any point along the length of a conventional optical fiber, which may have a single wave guide channel. There is an opportunity to strengthen the signal response to strain variation so that the strain related backscattered signal has a low sensitivity. The backscattered signal is also influenced by strain induced by torque and or bending forces exerted to the fiber, which decreases the selectivity of the measurement of axial strain variations along the length of the fiber.